1. Technical Field
The present invention relates to a detector system for a particle beam apparatus and a particle beam apparatus with such a detector system.
In particle beam apparatuses, such as for example scanning electron microscopes, the object is as a rule to detect, for image production, the particles emitted from an object by irradiation with a focused beam of primary particles. The particles emitted from the object can be divided into two groups: the particles back-scattered at the object by scattering, and the secondary particles emitted due to the excitation of the object by the primary particle beam. The particles or corpuscles back-scattered at the object then have nearly the energy of the primary particles at the object, while the secondary particles, for example the secondary electrons, have in relation to their energy a wide energy spectrum in the range of a few eV, clearly below the energy of the primary particles.
2. Background Art
A scanning electron microscope with a target structure arranged off-axis in the beam path is known from EP 0 661 727-A1. Furthermore, a magnetic and an electrostatic deflecting field are provided in the beam path, respectively act at right angles to the optical axis and are mutually orthogonal, and are excited with respect to each other in the manner of a Wien filter such that the action of the magnetic field and of the electrostatic field on the primary electron beam vanishes exactly. The back-scattered electrons and secondary electrons, propagating in exactly the opposite direction to the primary electron beam, are deflected away from the optical axis of the electron microscope by this combination of magnetic and electrostatic fields, so that they strike the target structure. The particles striking the target structure produce in their turn tertiary particles, conversion electrons, which are subsequently detected with the aid of a scintillation detector.
In order to detect only the back-scattered electrons in this detection system, a further electrostatic field perpendicular to the optical axis can be applied before the superposed magnetic and electrostatic field in the propagation direction of the secondary electrons; by means of it, the secondary electrons can be prevented from actually reaching the conversion electrode. Such an electrostatic field perpendicular to the optical axis can however only be applied without substantially affecting the primary beam when the secondary electrons also have only a very low energy with respect to the energy of the primary beam in the region of this deflecting field. In scanning electron microscopes in which the particles emitted by the specimen are accelerated back by an electrostatic lens into the beam guiding tube of the electron microscope, as is the case, for example, in the systems described in U.S. Pat. Nos. 4,831,266, 4,926,054, and DE-A1 198 29 476.4, and also the secondary electrons have at this place approximately the same energy as the primary electrons, due to this acceleration in the beam guiding tube, such a suppression of the secondary electrons without substantially affecting the primary particle beam is not possible.
In U.S. Pat. Nos. 5,900,629, 5,872,358, and the already mentioned DE-A1 198 28 476, further scanning electron microscopes are described, with a conversion diaphragm for the indirect detection of the secondary electrons and back-scattered electrons. However, a separate detection of the secondary electrons and of the back-scattered electrons is not considered in these documents.
A further detector system for a scanning electron microscope is described in EP-A1 0 917 178. This detector system has a diaphragm which is provided with a scintillation layer. The photons emitted by the scintillation layer due to bombardment with back-scattered electrons or secondary electrons are detected by means of a detector. With this system also, no separation is possible of the signal according to secondary electrons and back-scattered electrons.
A scanning electron microscope is described in EP-A1 0 917 177 which has a sequence of a magnetic, an electrostatic, and a second magnetic deflection field, for the spatial separation of the secondary electrons from the primary particles. Here also, no separation is possible of the signals produced by secondary electrons and by back-scattered electrons.